Semiconductor capacitor with praseodymium oxide as dielectric

ABSTRACT

In accordance with the invention there is provided a semiconductor capacitor having a first semiconductor layer which forms a first capacitor electrode and which includes silicon, a second capacitor electrode and a capacitor dielectric including praseodymium oxide between the capacitor electrodes, in which provided between the capacitor dielectric including praseodymium oxide and at least the first semiconductor layer including silicon is a first thin intermediate layer representing a diffusion barrier for oxygen. In particular the thin intermediate layer can include oxynitride.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is for entry into the U.S. national phase under §371for International Application No. PCT/EP03/04215 having an internationalfiling date of Apr. 23, 2003, and from which priority is claimed underall applicable sections of Title 35 of the United States Code including,but not limited to, Sections 120, 363 and 365(c), and which in turnclaims priority under 35 USC §119 to German Patent Application No. 10218 799.1 filed on Apr. 23, 2002.

TECHNICAL FIELD

The invention concerns a semiconductor capacitor with praseodymium oxideas dielectric and a process for the production of a thin oxynitridelayer on silicon.

BACKGROUND ART

It is no longer possible to imagine modern semiconductor technologywithout semiconductor capacitors. Important examples of use ofsemiconductor capacitors are dynamic random access memories (DRAM) inwhich the semiconductor capacitors are used as memory cells, and metaloxide semiconductor field effect transistors (MOSFETs) in which thesubstrate, the gate electrode and the gate oxide between the substrateand the gate electrode form a semiconductor capacitor.

Like all capacitors, it holds good for a semiconductor capacitor thatthe capacitance of the capacitor is proportional to the dielectricconstant of the dielectric between the capacitor electrodes and the areaof the capacitor electrodes as well as the reciprocal value of thespacing between the capacitor electrodes, that is to say the thicknessof the dielectric. Silicon oxide (SiO₂) is frequently used as thedielectric in the semiconductor art.

With the increasing reduction in the component size in the semiconductorart, the dimensions of the capacitor plates of semiconductor capacitors,for example the gate electrodes of MOSFETs, are also progressivelydecreasing. This means however that the capacitance of the semiconductorcapacitor is also reduced unless measures are taken to counteract that.

There are two possible ways of compensating for the reduction in thedimensions of the capacitor electrodes. The first option involvesreducing the thickness of the dielectric. For example in MOSFETs inwhich silicon oxide is typically used as the dielectric, that gives riseto problems, with gate lengths of less than 0.1 μm. The silicon oxidefor components with such short gate lengths would then have to bethinner than 1.5 nm. Such a thin silicon oxide however results in anincrease in the leakage current of the MOSFET. The leakage currentoccurs by virtue of electrons which tunnel through the thin gate oxidebetween the substrate and the gate electrode. The number of tunnellingelectrodes and thus the strength of the leakage current increasesexponentially with a progressively decreasing silicon oxide layerthickness. It is desirable however to minimise the leakage current of anMOSFET as the aim is to consume as little electrical power as possiblefor controlling the current between the drain electrode and the sourceelectrode.

The second possible way of compensating for the reduction in thecapacitor electrode area involves altering not the thickness of thedielectric but the dielectric constant thereof. If for examplepraseodymium oxide (Pr₂O₃) is used as the dielectric instead of siliconoxide, the capacitance of the capacitor can be markedly increased, withthe parameters involved being otherwise the same, by virtue of thepraseodymium oxide having a higher dielectric constant than siliconoxide. Silicon oxide has a dielectric constant of 3.9 whereaspraseodymium oxide has a dielectric constant of 30. This means that,with praseodymium oxide as the dielectric, the gate oxide can be thickerthan a dielectric of silicon oxide by the factor of 30 divided by 3.9.Therefore, with praseodymium oxide as the gate dielectric, the leakagecurrent can be drastically reduced in comparison with silicon oxide asthe dielectric.

The thickness of a silicon oxide layer which, with a constant area inrespect of the capacitor electrodes, affords the same capacitance as thepraseodymium oxide layer, is referred to hereinafter as the equivalentoxide layer thickness. With an increasing reduction in component size,that equivalent oxide layer thickness must be reduced in order tocompensate for the reduction in the capacitor electrode area. By meansof praseodymium oxide as the dielectric, it is possible to increase theactual oxide layer thickness in comparison with the equivalent oxidelayer thickness, and thus reduce the tunnelling leakage current.

Praseodymium oxide is typically deposited by vapor deposition of Pr₆O₁₁on silicon. In that case, a mixed oxide of the form(PrO₂)_(x)(SiO₂)_(1−x) with O<×<1, typically in non-stoichiometric form,is formed between the silicon and the praseodymium oxide (Pr₂O₃).Thermal process steps following the deposition procedure additionallyresult in further spreading of the mixed oxide, in particular the SiO₂component.

The mixed oxide has a lower dielectric constant than the purepraseodymium oxide, whereby the equivalent oxide layer thickness of thedielectric is increased in comparison with a pure praseodymium oxidedielectric. The mixed oxide therefore worsens the electrical propertiesof the dielectric and thus the semiconductor capacitor, morespecifically to a greater degree in proportion to an increasingthickness of the mixed oxide.

DISCLOSURE OF THE INVENTION

Therefore an object of the present invention is to develop asemiconductor capacitor with praseodymium oxide as the dielectric, insuch a way that it has improved electrical properties.

That object is attained by a semiconductor capacitor according to theinvention.

In accordance with the invention, to attain the above-indicated object,there is provided a semiconductor capacitor having a first semiconductorlayer which forms a first capacitor electrode and which includessilicon, a second capacitor electrode and a capacitor dielectricincluding praseodymium oxide between the capacitor electrodes, in whichprovided between the capacitor dielectric including praseodymium oxideand at least the first semiconductor layer including silicon is a firstthin intermediate layer representing a diffusion barrier for oxygen.

In that respect the term layer is to be interpreted as meaning not justa material region which extends in parallel relationship with thesurface of a semiconductor substrate. A layer in accordance with theinvention can also extend perpendicularly to or at any other angles tothe substrate surface. In particular the semiconductor capacitor can bein the form of a vertical or lateral semiconductor capacitor.

The first thin intermediate layer is a diffusion barrier which preventsoxygen reaching and oxidising the silicon surface in thermal processsteps which follow the step of depositing the praseodymium oxide-bearinglayer. It is possible in that way to prevent the SiO₂ component of the(PrO₂)_(x)(SiO₂)_(1−x) mixed oxide growing uncontrolledly as aconsequence of the reaction of the silicon with oxygen, and worseningthe dielectric constant of the capacitor dielectric. In addition thethin intermediate layer can also function as a shielding means for thesubstrate material in relation to external influences.

In accordance with a further configuration of the invention the firstthin intermediate layer includes silicon oxynitride (this can occur invarious stoichiometric ratios, for example SiON), hereinafter referredto for brevity as oxynitride, as a material for inhibiting oxygendiffusion. The thin intermediate layer including oxynitride, besidesinhibiting oxygen diffusion, permits structural and electronicadaptation of the praseodymium oxide to the silicon-bearingsemiconductor layer and can therefore be viewed as an adaptation layer.It reduces the trap density, that is to say the density of electricalstate levels in the band gap of the semiconductor material, which trapfree electrons and can thus produce unwanted charge states. In additionthe oxynitride-containing intermediate layer does not impedeheteroepitaxial growth (epitaxial growth of a material on a substrateconsisting of another material) of praseodymium oxide on thesilicon-containing semiconductor material, so that monocrystallinegrowth of the praseodymium oxide is possible.

Alternatively or in addition, it is possible as the first thinintermediate layer on the praseodymium oxide layer to apply a thin Tilayer which serves itself as a gate electrode or however as anintermediate layer between the gate dielectric and the gate electrode.Titanium as a particularly reactive transition metal can chemically bindthe excess oxygen in the praseodymium oxide layer or the oxygen whichpossibly penetrates into that layer from the exterior. Such a titaniumlayer can protect the layer stack and thus the interface in relation tooxygen, irrespective of whether it originates from the atmosphere orwhether it takes effect during a technological process step.

Preferably the thickness of the first thin intermediate layer is 0.5 nmor less. The thinner the first thin intermediate layer is, thecorrespondingly less does it influence, as part of the capacitordielectric, the electrical properties of the capacitor. The thickness ofthe first thin intermediate layer can be reduced to such an extent asthe production processes allow, as long as the effect thereof as adiffusion-inhibiting layer does not suffer by that.

In an embodiment of the invention the second capacitor electrode isformed from a second semiconductor layer. In addition, a second thinintermediate layer is present between the second semiconductor layer andthe capacitor dielectrode containing praseodymium oxide. Uncontrolledchemical reactions between the capacitor dielectric and the secondsemiconductor layer can be avoided by means of the second thinintermediate layer.

Particularly when the second semiconductor layer includes silicon, it isadvantageous if the second thin intermediate layer includes oxynitrideor silicon oxide in order to inhibit oxygen diffusion or chemicalreactions. In addition, with silicon oxide as the thin secondintermediate layer, it is possible, when applying polycrystallinesilicon (polysilicon) as the second semiconductor layer, for example asthe gate electrode of an MOSFET, to contact the capacitor dielectric inconventional manner, that is to say as in the case of a capacitordielectric of silicon oxide. The thickness of the second thinintermediate layer, like that of the first thin intermediate layer, ispreferably also 0.5 mm or less.

Advantageously the oxynitride of the thin intermediate layers has aconcentration ratio of oxygen to nitrogen (O/N concentration ratio) of1:1.

Semiconductor capacitors in accordance with the invention can beparticularly advantageously used in memory cells for dynamic randomaccess memories (DRAM) or as a gate capacitance in field effecttransistors.

In accordance with the invention there is also provided a process forthe production of a thin oxynitride layer on a semiconductor material,in which firstly there is applied to the semiconductor material a verythin silicon oxide layer which is then subjected to heat treatment fornitriding purposes in a nitrogen atmosphere (N₂ atmosphere) at 800° C.

In addition there is also provided a process for the production of athin oxynitride layer on silicon, in which the silicon surface isexposed at 800° C. to a nitrogen monooxide atmosphere (NO atmosphere) ora laughing gas atmosphere (N₂O atmosphere).

Thin oxynitride layers for semiconductor capacitors according to theinvention can advantageously be produced with the processes according tothe invention. In particular, with the processes according to theinvention, it is possible to produce oxynitride layers of such smallthicknesses as 0.2 nm and with an O/N concentration ratio of 1:1.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-described and further features and advantages of the inventionare described in greater detail hereinafter by means of embodiments byway of example with reference to the accompanying drawings in which:

FIG. 1 shows a portion of a first embodiment of the semiconductorcapacitor according to the invention, and

FIG. 2 shows a portion of a second embodiment of the semiconductorcapacitor according to the invention.

FIG. 1 shows a portion of the layer sequence of a semiconductorcapacitor according to the invention with reference to the example ofthe gate capacitance of an MOSFET.

DETAILED DESCRIPTION OF THE INVENTION

The gate capacitance of an MOSFET of which a portion is shown in FIG. 1includes a channel region which is formed by a silicon substrate 1 andwhich forms the first capacitor electrode and a polysilicon gateelectrode 3 which forms the second capacitor electrode. Disposed betweenthe silicon substrate 1 and the polysilicon gate electrode 3 is a gatedielectric 5 of praseodymium oxide (Pr₂O₃) as the capacitor dielectric.Provided at the interface of the gate dielectric 5, which is towards thesubstrate 1, is a mixed oxide layer which originates from the productionprocess and which is of the form (PrO₂)_(x)(SiO₂)_(1−x), wherein x canassume values in the range of greater than zero and less than one. Inaddition disposed between the mixed oxide 7 and the silicon substrate 1is a thin oxynitride layer 9 which serves as a diffusion barrier foroxygen and which prevents oxygen from reaching the silicon surface ofthe substrate 1 through the praseodymium oxide 5 and oxidising thesilicon surface during heat treatment steps which follow the deposit ofthe praseodymium oxide layer 5 on the silicon substrate 1. Theoxynitride layer 9 is produced in the production process prior to thepraseodymium oxide layer 5 being deposited.

The diffusion-inhibiting oxynitride layer 9 according to the inventionmakes it possible to substantially prevent oxygen diffusion to thesurface of the silicon substrate and thus suppress uncontrolled growthof the mixed oxide 7. Therefore the thickness of the mixed oxide layer 7can be reduced in comparison with the state of the art. At the same timethe thin oxynitride layer 9 reduces the trap density, that is to say thedensity of electrical state levels in the band gap of the semiconductormaterial, which trap free electrons and can thus produce unwanted chargestates.

Instead of an oxynitride layer 9 it is also possible to use a layercomprising another material if that material inhibits diffusion ofoxygen and preferably also does not adversely affect heteroepitaxialgrowth of the praseodymium oxide on the substrate.

A thin oxynitride layer on silicon can be implemented for example by aprocedure whereby firstly a very thin silicon oxide layer is depositedon the silicon substrate and the deposited silicon oxide layer is thensubjected to heat treatment in a nitrogen atmosphere at 800° C. In thatheat treatment procedure the thin silicon oxide layer is nitrided.Alternatively the thin oxynitride layer can be implemented on silicon bya procedure whereby the silicon surface of the substrate is exposed at800° C. to an NO or N₂O atmosphere. Both processes make it possible toproduce oxynitride layers with a layer thickness of 0.2 nm and an O/Nconcentration ratio of 1:1.

A second embodiment by way of example is illustrated in FIG. 2. Thesecond embodiment differs from the first embodiment only in that asilicon oxide layer 11 is arranged as the thin intermediate layer,between the gate dielectric 5 and the polysilicon gate electrode 3.Preferably that silicon oxide layer 11 is a thin SiO₂ layer of athickness of 0.5 nm or less. In particular the thickness can also beless than 0.3 nm. The silicon oxide layer 11 serves as an interfacelayer which permits both conventional contacting, that is to saycontacting as in the case of a gate dielectric consisting of siliconoxide, and also chemical reactions between the Pr₂O₃ gate dielectric andthe polysilicon of the gate electrode during thermal process steps whichfollow deposit of the polysilicon gate electrode 3.

Instead of a silicon oxide layer as the thin intermediate layer 11 it isalso possible to provide a thin oxynitride layer between the polysilicongate electrode 3 and the gate dielectric 5. The thickness thereof ispreferably also 0.5 nm or less and in particular less than 0.3 nm. Theeffects achieved with the oxynitride layer as the second intermediatelayer 11 are similar to those which can be achieved with the siliconoxide layer. Thus, both oxynitride and also silicon oxide, afterproduction of the praseodymium oxide layer, can serve as a protectivelayer for the praseodymium oxide and as a contacting aid when contactingthe gate electrode.

In the embodiments illustrated hitherto the gate electrode 3 comprisespolysilicon. It is however also possible for the gate electrode 3 to beproduced from another material, for example amorphous silicon orpolycrystalline or amorphous silicon germanium (SiGe). The gateelectrode can include carbon or oxygen for suppressing dopant diffusion.In addition gate electrodes of other conventional materials, for examplemetallic gate electrodes or gate electrodes comprisingmetal-semiconductor compounds are also possible.

The substrate is also not restricted to a silicon substrate. It is alsopossible to use for example a silicon germanium substrate, a silicongermanium substrate with carbon or oxygen as a diffusion inhibitor or asilicon substrate with carbon or oxygen as a diffusion inhibitor.Likewise the substrate does not need to be present in a (001) crystalorientation.

In a further embodiment (not illustrated in the Figures) thesemiconductor capacitor according to the invention serves as a memoryelement for a memory cell of a dynamic random access memory (DRAM). Apraseodymium oxide layer is arranged as a dielectric between twocapacitor electrodes comprising a semiconductor material, for examplecrystalline, polycrystalline or amorphous silicon or silicon germanium,in each case with or without carbon or oxygen. Disposed between thepraseodymium oxide and the semiconductor material of the capacitorelectrodes is a silicon oxide layer or an oxynitride layer with which itis possible to prevent unwanted reactions between the Pr₂O₃ dielectricand the semiconductor material of the capacitor electrodes.

Configurations of the memory element are also possible, in which anintermediate layer is present only between one of the capacitorelectrodes and the praseodymium oxide layer.

1. A semiconductor capacitor having a first semiconductor layer whichforms a first capacitor electrode (1) and which includes silicon, asecond capacitor electrode (3) and a capacitor dielectric (5) includingpraseodymium oxide between the capacitor electrodes (1, 3),characterised in that provided between the capacitor dielectric (5)including praseodymium oxide and at least the first semiconductor layer(1) including silicon is a first thin intermediate layer (9) serving asa diffusion barrier for oxygen.
 2. A semiconductor capacitor as setforth in claim 1 characterised in that the first thin intermediate layer(9) includes oxynitride or titanium.
 3. A semiconductor capacitor as setforth in claim 2 wherein the oxynitride of the first or the second thinintermediate layer (9, 11) has a concentration ratio of oxygen tonitrogen of 1:1.
 4. A semiconductor capacitor as set forth in claim 1 orclaim 2 wherein the thickness of the first thin intermediate layer (9)is 0.5 nm or less.
 5. A semiconductor capacitor as set forth in claim 1wherein the second capacitor electrode (3) is formed from a secondsemiconductor layer and there is a second thin intermediate layer (11)between the second semiconductor layer and the capacitor dielectric (5)and the second semiconductor layer includes praseodymium.
 6. Asemiconductor capacitor as set forth in claim 5 wherein the second thinintermediate layer (11) includes oxynitride.
 7. A semiconductorcapacitor as set forth in claim 6 wherein the oxynitride of the first orthe second thin intermediate layer (9, 11) has a concentration ratio ofoxygen to nitrogen of 1:1.
 8. A semiconductor capacitor as set forth inclaim 5 wherein the second thin intermediate layer (11) includes siliconoxide.
 9. A semiconductor capacitor as set forth in claim 5 wherein thethickness of the second thin intermediate layer (11) is 0.5 nm or less.10. A memory cell for a dynamic random access memory, which includes asemiconductor capacitor as set forth in claim
 1. 11. A field effecttransistor comprising a substrate (1), a gate oxide layer (5) and a gateelectrode (3), which includes a semiconductor capacitor as set forth inclaim 1, wherein the substrate (1) forms the first capacitor electrode,the gate electrode (3) forms the second capacitor electrode and the gateoxide (5) forms the capacitor dielectric.
 12. A semiconductor capacitoras set forth in claim 1 wherein the first thin intermediate layer (9) istitanium.